Various application programs can be installed in a terminal device such as a personal computer, to provide a variety of functionality. Such application programs can be installed by means of storage media such as a CD-ROM, or via a server on a network such as the Internet; alternatively, application programs may be preinstalled by a manufacturer of a terminal, or installed by a retail seller of a terminal who has a license contract with the application program manufacturer.
However, not all sources of application programs can be trusted; malicious individuals may, for example, make available damaging or dangerous software via servers on the Internet.